In the field of electrophotographic color printing, prior art methods of reconstructing a color image have employed processes wherein a series of single color images are first written and developed in sequence on a photoconductive member and then transferred from the photoconductive member via a transfer member, such as a transfer belt or transfer drum, to a print media, such as paper. The primary colors of cyan, yellow, magenta and black (C, Y, M, and K) are commonly used in laser printers for this purpose, and the C, Y, M, and K images are superimposed one upon another on paper to form a composite color image which is then fused or fixed into the paper. This type of electrophotographic or laser printing process is disclosed and claimed in co-pending application Ser. No. 515,946 of C. S. Chan et al filed Apr. 27, 1990, assigned to the present assignee and incorporated herein by reference.
In comparison to the well developed monochromatic image development and transfer processes in the field of electrophotography wherein a single black and white image is first formed on a photoconductive drum and then transferred in a single pass process and fused into the paper, this type of multiple color and multiple pass electrophotographic printing process presents many completely new and different technical problems and challenges to workers in this relatively new and rapidly developing art. More particularly, instead of having to be concerned with only the transfer of a single color image from a photoconductive drum by a transfer drum to paper and fused therein, there are instead now four color images of cyan, yellow, magenta and black in this multiple color-multiple pass process that have to be transferred from the photoconductive drum via the transfer medium to the paper. These requirements greatly increase the complexity of the overall color printing process as a result of the multiple image color development, color mixing and the handling of the four (C, Y, M, and K) non-fused wet toners at one time which is involved in the above color image superimposition processes.
Previously, color and multiple image electrophotographic processes have been developed wherein the above primary color images are fused or fixed into the print medium before a subsequent primary color image is superimposed thereon. Examples of such processes are disclosed in U.S. Pat. No. 4,783,681 issued to Tanaka et al and in U.S. Pat. No. 4,799,086 issued to Koike et al, both assigned to Canon of Japan. However, these prior systems are rather complex mechanically and neither of these prior systems provide for paper shrinkage compensation during the media fusion process thereon. In addition, the paper registration compensation process disclosed in Koike et al U.S. Pat. No. 4,799,086 employs mechanical means rather than electronic image control compensation for the subsequently printed images, thereby making its registration accuracy less than completely reliable in all cases. In addition, the construction of the apparatus in Koike et al is inherently more expensive than the image control compensation system of the present invention to be described herein.